Thiol compounds such as trimethylolpropane tris(3-mercaptopropionate) and pentaerythritol tetrakis(3-mercaptopropionate) are used widely in various applications in every kind of industrial field. Among them, the most general one is use of them for photosensitive compositions.
Photosensitive compositions are used in various fields including many fields such as printing plates, color proofs, color filters, solder resists, and photocuring ink. In particular, in recent years, room temperature, fast-curing and solventlessness, which are the most characteristic properties of photocuring, have received attention in various fields including these applications from the viewpoints of environmental pollution, energy saving, safety in working, production costs and the like and many studies and developments have been made on photosensitive compositions. In the development of color filters, studies on pigment dispersion type resist for color filters have been under way for the purpose of increasing productivity and achieving high precision. Also, in color proofs and printing plates, development has been under way for the purpose of achieving high speed and high precision in plate making. Further, solder resists for printed boards have been studied.
In these applications, there has been an increasing demand for photosensitive compositions and those which cure at lower energy, those which cure at higher rates, those which can form more precise patterns, those which have deeper curing depth, and those which have higher storage stability performance have been sought. Photosensitive compositions are mainly composed of a photopolymerization initiator composition, a compound having an ethylenically unsaturated bond which cures by a polymerization reaction and various kinds of additives, and the kinds of the components depend on use to which the photosensitive composition is applied.
Note that in the present invention, by the “photopolymerization initiator composition” is meant a composition containing at least one compound which participates in the initiation reaction of photopolymerization, such as a compound that generates a radical, an anion, or a cation by light, a compound having a chain transfer action, or a compound having sensitization activity.
The compounds which constitute the photopolymerization initiator composition are selected by their photosensitive wavelengths and polymerization initiating properties. The compound having an ethylenically unsaturated bond and the additives are selected by polymerizability and physical properties of a desired cured product. They are combined and used as a photosensitive composition. However, some compounds having an ethylenically unsaturated bond and some additives occasionally cause problems as follows in the photosensitive compositions. (1) Sufficient energy for initiating photopolymerization is not obtained; (2) storage stability is not obtained; (3) since irradiation light does not reach deep enough in a desired cured product due to the thickness thereof, curing proceeds only insufficiently; and (4) oxygen inhibition occurs at the portion where the photosensitive composition contacts the air. For these problems, various measures have been taken; for example, irradiation of greater light energy, use of an excess amount of a photopolymerization initiator and provision of an oxygen shielding membrane. Also for energy saving and reduction in production cost, a photosensitive composition having more excellent photocurability and storage stability is desired.
As for the photopolymerization initiator composition containing a thiol compound, Japanese Patent Application Laid-open No. Hei 10-253815 discloses a photopolymerizable composition containing a polyfunctional thiol and an initiator selected from a group consisting of a biimidazole compound, a titanocene compound, a triazine compound and an oxazole compound. Japanese Patent Application Laid-open No. 2000-249822 (EP1031579A2, U.S. Pat. No. 6,455,207) discloses a photopolymerization initiator containing a sensitizer, an organic boron complex and a compound having a mercapto group. However, the attempt of achieving high sensitivity by use of such a polyfunctional thiol compound causes a problem of costing the storage stability.
The inventors of the present invention have found that in order to obtain a photosensitive composition which has high sensitivity and excellent photocurability as well as excellent storage stability, selection of a photopolymerization initiator composition is important; in particular, selection of a thiol compound used as one component of the photopolymerization initiator composition is important.
Therefore, an object of the present invention is to provide a photopolymerization initiator composition having high sensitivity and excellent storage stability, a photosensitive composition containing the photopolymerization initiator composition, and a novel thiol compound suitable for the photopolymerization initiator composition.